2,3-Unsaturated carboxylic acids and esters can be prepared from the reaction of a formaldehyde (H2CO) source and a saturated carboxylic acid or ester containing one less carbon atom. Thus, acrylic and methacrylic acid derivatives can be prepared from the condensation of a formaldehyde source with acetic or propionic acid derivatives, respectively. The reaction produces one equivalent of water for each equivalent of carboxylic acid derivative reacted.
Although a number of catalysts have been proposed for this reaction, catalysts containing acidic vanadium and phosphorus oxides are among the most efficient, especially when a third component such as titanium or silicon is present in the catalyst. Water, however, tends to inhibit the condensation reaction with these catalysts. The use of formalin—which typically contains about 37 weight percent formaldehyde in water—as a starting material, therefore, is less efficient. Methanol can also be an inhibitor for the condensation reaction, and, since formalin can also contain methanol, the efficiency can be further lowered. When a carboxylic acid is the reactant, the presence of methanol in formalin can create a mixture of acids and methyl esters. And when an ester is the reactant, the water in formalin can create a mixture of acids and esters.
Industrial grade aqueous formaldehyde contains about 55 weight percent formaldehyde. It is relatively inexpensive and, therefore, is an economical source of this reactant. Thus, there is a need in the art for catalysts that are capable of condensing formaldehyde with alkanoic acids or esters in the vapor phase and that are tolerant of water in the feedstock. Ideally, such catalysts would also provide a high conversion of formaldehyde along with a high selectivity to the acrylic product.
The conventional process for these aldol condensation reactions combines a formaldehyde source, such as trioxane, with a carboxylic acid to form water, the 2,3-unsaturated carboxylic acid, and formaldehyde. The formaldehyde can react with itself at any time during the reaction to form paraformaldehyde. This by-product formation of paraformaldehyde can contribute to yield losses and increased maintenance costs as the paraformaldehyde deposits on equipment and piping.
Conventional Feed

The problems caused by paraformaldehyde create the need to make 2,3-unsaturated carboxylic acids without producing significant paraformaldehyde. One solution is to introduce a methylene unit by an alternative feed that does not utilize or produce formaldehyde which can polymerize to paraformaldehyde. A methylene dialkanoate feed can be used as such an alternative feed.
Methylene Dialkanoate Feed

The use of a methylene dialkanoate feed can also lead to improved space time yield (STY) while operating at decreased temperatures, even in the presence of extraneous water when compared to the conventional reaction process. These reaction improvements come as a surprise since acrylic acid production from conventional feeds comprising acetic acid and formaldehyde (as trioxane) are negatively impacted by water and reduced temperature. The practical utility of these benefits are increased catalyst lifetime and maintained STY when water is introduced to the reaction system from impure gas lines or generated via by-product chemistry.
Although the V—Ti—P catalysts of the present invention function with the presence of water, improved STY can be seen by attenuating the effects of water. One approach to reduce the presence of water in the feed is to replace aqueous formaldehyde with anhydrous formaldehyde (trioxane, C3H6O3). Despite this replacement, the molar addition of trioxane with acetic acid still includes one mole of a latent molecular water, thereby limiting the maximum attainable rate. To further offset the effect of water, methylene dialkanoates, such as methylene diacetate (MDA) and methylene dipropionate (MDP) can be synthesized from formaldehyde and utilized as a feed towards the production of acrylic acid and methacrylic acid, respectively. These methylene dialkanoates are molecularly equivalent to one mole of formaldehyde and two moles of the corresponding carboxylic acid but without the latent molecular water (i.e. one mole of latent water is not produced). MDA and MDP form acrylic acid and methacrylic acid, respectively, over the V—Ti—P catalyst at a surprisingly high reaction rate and yield.
Vanadium-titanium-phosphorus (V—Ti—P) mixed oxides are the best known catalysts for generating acrylic acid from the condensation of formaldehyde and acetic acid. But the preparation of these catalysts can be dangerous and is not amenable to scale-up. Typically, the titanium component is incorporated into these catalysts by first hydrolyzing liquid titanium chloride. This step, unfortunately, generates large quantities of hydrochloric acid fumes. Thus, there is also a need in the art for methods of preparing V—Ti—P mixed oxide catalysts that are safer and more amenable to industrial production.
The reactions catalyzed by vanadium-titanium-phosphorus catalysts usually produce few undesired side reactions and offer high yields. However, one of the drawbacks with V—Ti—P condensation catalysts is they can quickly deactivate during the reaction due to coking on the catalyst. Although the V—Ti—P catalyst can typically be regenerated by removing the coke at an elevated temperature in the presence of oxygen, the lifetime of the reactive catalyst between regenerations can be limited and short. It has been shown that co-feeding oxygen can help stabilize the condensation catalyst over a short period of time, i.e. 8 hours. However, inferior yields to acrylic acid were also observed. Thus, there is a need to extend the reactive lifetime of the V—Ti—P catalyst while maintaining the high conversion and selectivity by co-feeding oxygen at certain concentrations to the reactor.
The present invention addresses these needs as well as others that will be apparent from the following description and claims.